U.S. patent application number 11/094074 was filed with the patent office on 2006-10-12 for dual connection power line parameter analysis method and system.
Invention is credited to Michael L. Gasperi, David L. Jensen, David T. Rollay.
Application Number | 20060229834 11/094074 |
Document ID | / |
Family ID | 37084139 |
Filed Date | 2006-10-12 |
United States Patent
Application |
20060229834 |
Kind Code |
A1 |
Gasperi; Michael L. ; et
al. |
October 12, 2006 |
Dual connection power line parameter analysis method and system
Abstract
A method and apparatus is disclosed for determining the power
line parameters of a system. Specifically, there is provided a
method comprising perturbing a voltage waveform through a first
connection, measuring a characteristic of the perturbation through
a second connection, and calculating a line impedance based on the
characteristic of the perturbation.
Inventors: |
Gasperi; Michael L.;
(Racine, WI) ; Jensen; David L.; (Barneveld,
WI) ; Rollay; David T.; (Franklin, WI) |
Correspondence
Address: |
ROCKWELL AUTOMATION, INC./(AT)
ATTENTION: SUSAN M. DONAHUE
1201 SOUTH SECOND STREET
MILWAUKEE
WI
53204
US
|
Family ID: |
37084139 |
Appl. No.: |
11/094074 |
Filed: |
March 30, 2005 |
Current U.S.
Class: |
702/65 |
Current CPC
Class: |
G01R 27/16 20130101 |
Class at
Publication: |
702/065 |
International
Class: |
G01R 27/00 20060101
G01R027/00 |
Claims
1. A system for determining power line impedance comprising: a
plurality of bus bars coupled to a power source; and a first set of
connection points electrically coupled to the bus bars; a second
set of connection points electrically coupled to the bus bars; a
voltage perturbation circuit coupled to the plurality of bus bars
through the first set of connection points and configured to
generate a resonant ring; a voltage measurement circuit coupled to
the plurality of bus bars through the second set of connection
points and configured to measure a frequency of the resonant ring;
and a processing circuit for determining inductive and resistive
components of an impedance of the power line based on the frequency
of the resonant ring.
2. The system of claim 1, further comprising an incident energy
calculation module for calculating an incident energy based at
least partially on the impedance and a plurality of inputs stored
in a memory.
3. The system of claim 2, comprising an input device configured to
store the plurality of inputs in the memory.
4. The system of claim 1, further comprising a display panel for
displaying one of more power line parameters.
5. The system of claim 1, wherein the processing circuit is remote
from the voltage perturbation circuit and the voltage measurement
circuit.
6. A method comprising: measuring a first voltage of an ac waveform
applied to an electrical line over a first electrical contact;
draining current from the electrical line over a second electrical
contact; measuring a second voltage of the ac waveform over the
first electrical contact during a droop in voltage resulting from
the current drain; removing the current drain to cause a resonant
ring in the voltage in the electrical line; measuring a frequency
of the resonant ring over the first electrical contact; and
computing the line impedance based upon the measured first and
second voltages and the frequency of the resonant ring.
7. The method of claim 6, comprising computing inductive and
resistive components of the line impedance.
8. The method of claim 6, comprising periodically sampling voltage
during the measuring steps and storing values representative
thereof.
9. The method of claim 8, wherein measuring the voltages includes
measuring peak voltages based upon the stored sampled voltage
values.
10. The method of claim 6, wherein the electrical line carries
single phase power.
11. The method of claim 6, comprising computing an incident energy
based on the line impedance.
12. The method of claim 11, comprising displaying the incident
energy.
13. The method of claim 6, comprising computing a flash protection
boundary based on the line impedance.
14. A method comprising: coupling a first circuit to a voltage
source via a first electrical contact, wherein the first circuit is
configured to perturb the ac waveform of the voltage source;
coupling a second circuit to the voltage source via a second
electrical contact, wherein the second circuit is configured to
measure characteristics of the ac waveform and determine a line
impedance of the voltage source based on the characteristics of the
ac waveform; and initiating a routine to determine the line
impedance.
15. The method of claim 14, comprising initiating a routine to
determine a level of personal protective equipment, wherein the
routine to determine the level of personal protective equipment
employs the line impedance.
16. The method of claim 15, comprising displaying indicia
representative of the personal protective equipment associated with
the determined level of personal protective equipment.
17. A method comprising: perturbing a voltage waveform through a
first connection; measuring a characteristic of the perturbation
through a second connection; and calculating a line impedance based
on the characteristic of the perturbation.
18. The method of claim 17, wherein measuring the characteristic
comprises measuring a frequency of a resonant ring.
19. The method of claim 17, wherein measuring the characteristic
comprises measuring a characteristic for each phase of a three
phase ac waveform.
20. The method of claim 17, comprising determining an incident
energy using the line impedance.
21. The method of claim 17, comprising determining a PPE level
using the line impedance.
22. A motor control center comprising: a plurality of bus bars
coupled to a power source; and a first set of stabs electrically
coupled to the bus bars; a second set of stabs electrically coupled
to the bus bars; a voltage perturbation circuit coupled to the
plurality of bus bars through the first set of stabs and configured
to generate a resonant ring; and a voltage measurement circuit
coupled to the plurality of bus bars through the second set of
stabs and configured to measure a frequency of the resonant
ring.
23. The motor control center of claim 22, further comprising a
processing circuit for determining inductive and resistive
components of an impedance of the power line based on the frequency
of the resonant ring.
24. The motor control center of claim 22, further comprising an
incident energy calculation module for calculating an incident
energy based at least partially on the impedance.
25. The motor control center of claim 22, wherein the motor control
center is configured to communicate the frequency of the resonant
ring to a remote monitoring device.
Description
BACKGROUND
[0001] The present technique relates generally to the field of
electrical distribution. Specifically, the invention relates to
techniques for determining the impedance parameters of electrical
power, for determining incident energy, for determining a flash
protection boundary, and for determining a level of personal
protective equipment ("PPE") that may be required or advisable
based upon the available energy and similar considerations.
[0002] Systems that distribute electrical power for residential,
commercial, and industrial uses can be complex and widely divergent
in design and operation. Electrical power generated at a power
plant may be processed and distributed via substations,
transformers, power lines, and so forth, prior to receipt by the
end user. The end user may receive the power over a wide range of
voltages, depending on availability, intended use, and other
factors. In large commercial and industrial operations, the power
may be supplied as three phase ac power (e.g., 208 to 690 volt ac,
and higher). Power distribution and control equipment then
conditions the power and applies it to loads, such as electric
motors and other equipment. In one exemplary approach, collective
assemblies of protective devices, control devices, switchgear,
controllers, and so forth are located in enclosures, sometimes
referred to as "motor control centers" or "MCCs". Though the
present techniques are discussed in the context of MCCs, the
techniques may apply to power management systems in general, such
as switchboards, switchgear, panelboards, pull boxes, junction
boxes, cabinets, other electrical enclosures, and distribution
components.
[0003] A typical MCC may manage both application of electrical
power, as well as data communication, to the loads, such loads
typically including various machines or motors. A variety of
components or devices used in the operation and control of the
loads may be disposed within the MCC. Exemplary devices contained
within the MCC are motor starters, overload relays, circuit
breakers, and solid-state motor control devices, such as variable
frequency drives, programmable logic controllers, and so forth. The
MCC may also include relay panels, panel boards, feeder-tap
elements, and the like. Some or all of the devices may be disposed
within units sometimes referred to as "buckets" that are mounted
within the MCC. The MCC itself typically includes a steel enclosure
built as a floor mounted assembly of one or more vertical sections
containing the buckets.
[0004] The MCC normally contains power buses and wiring that supply
power to the buckets and other components. For example, the MCC may
house a horizontal common power bus that branches to vertical power
buses within the MCC. The vertical power buses, known as bus bars,
then extend the common power supply to the individual buckets.
Other large power distribution equipment and enclosures typically
follow a somewhat similar construction, with bus bars routing power
to locations of equipment within the enclosures.
[0005] To electrically couple the buckets to the vertical bus, and
to simplify installation and removal, the buckets may comprise
electrical connectors or clips, known as stabs. To make the power
connection, the stabs engage (i.e., clamp onto) the bus bars. For
three phase power, there may be at least three stabs per bucket to
accommodate the three bus bars for the incoming power. It should be
noted that though three phase ac power is primarily discussed
herein, the MCCs may also manage single phase or dual phase ac
power, as well as dc power (e.g., 24 volt dc power for sensors,
actuators, and data communication). Moreover, in alternate
embodiments, the individual buckets may connect directly to the
horizontal common bus by suitable wiring and connections.
Similarly, in contexts other than MCCs, the structures described
herein will, of course, be adapted to the system, its components,
and any enclosures that house them.
[0006] A problem in the operation of MCCs and other power
management systems, such as switchboards and panelboards, is the
occurrence of arcing (also called an arc, arc fault, arcing fault,
arc flash, arcing flash, etc.) which may be thought of as an
electrical conduction or short circuit across the air between two
conductors. Initiation of an arc fault may be caused by a loose
connection, build-up of foreign matter such as dust or dirt,
insulation failure, or a short-circuit between the two conductors
(e.g., a foreign object establishing an unwanted connection between
phases or from a phase to ground) which causes the arc. Once
initiated, arcing faults often typically proceed in a substantially
continuous manner until the power behind the arc fault is turned
off. However, arcing faults can also comprise intermittent failures
between phases or phase-to-ground. In either case, the result is an
intense thermal event (e.g., temperatures up to 35,000.degree. F.)
causing melting or vaporization of conductors, insulation, and
neighboring materials.
[0007] The energy released during an arcing fault is known as
incident energy. Incident energy is measured in energy per unit
area, typically Joules per square centimeter (J/cm.sup.2). Arcing
faults can cause damage to equipment and facilities and drive up
costs due to lost production. More importantly, the intense heat
generated by arcing faults has led to the establishment of
standards for personal protective equipment ("PPE") worn by service
personnel when servicing electrical equipment.
[0008] There are five levels of PPE numbered from 0 to 4. Whereas,
level 0 PPE comprises merely a long sleeved shirt, long pants, and
eye protection, level 4 PPE comprises a shirt, pants, a flame
retardant overshirt and overpants, a flash suit, a hard hat, eye
protection, flash suit hood, hearing protection, leather gloves,
and leather work shoes. PPE levels 1-3 comprise increasing amounts
of protective clothing and equipment in increasing greater amounts
between levels 0 and 4. As such, the higher the PPE level, the more
protective clothing or equipment a person will put on (referred to
as "donning") or take off (known as "doffing") before servicing the
equipment. Accordingly, the time to donn and doff the protective
equipment increases as the PPE level increases. For example whereas
it may take less than a minute to donn or doff level 1 PPE, it may
take 20 minutes or more to donn or doff level 4 PPE. These donning
and doffing times can directly affect productivity. As such, it is
advantageous to accurately determine the potential incident energy
of a potential arc flash so that the appropriate level of PPE.
[0009] Many other uses and applications exist for information
relating to incident energy, and other power line electrical
parameters. These include, but are not limited to, the sizing and
design of filters, the commissioning and design of motor drives and
other equipment, the monitoring of power lines and components for
degradation and failure, and so forth.
[0010] Conventional methods for determining power line parameters
and PPE levels rely on approximation techniques or require complex,
extensive modeling of electrical equipment. There is a need in the
art for improved techniques for determining the incident energy.
There is a particular need for a technique that would permit the
accurate determination and communication of incident energy, the
determination of flash protection boundaries, and the determination
and communication of PPE levels that correspond to a particular
incident energy.
BRIEF DESCRIPTION
[0011] The present invention provides novel techniques for
determining the incident energy of a potential arc flash in an
electrical device, for determining a flash protection boundary, for
determining a PPE level, and for communicating such determinations
to users and service personnel. The techniques can be used on
single-phase and three-phase applications with little modification.
Moreover, the technique can be implemented in permanent (i.e.,
hard-wired) circuitry, or can be part of portable or even hand-held
devices used to determine the incident energy on a periodic or
sporadic basis. Still further, the technique may be implemented in
a stand-alone embodiment or in a distributed network.
DRAWINGS
[0012] These and other features, aspects, and advantages of the
present invention will become better understood when the following
detailed description is read with reference to the accompanying
drawings in which like characters represent like parts throughout
the drawings, wherein:
[0013] FIG. 1 is a diagrammatical representation of a power line
impedance measurement system in accordance with aspects of the
present technique, applied to a single-phase application;
[0014] FIG. 2 is a somewhat more detailed view of certain of the
circuitry of the power line impedance measurement system of FIG.
1;
[0015] FIG. 3 is a diagrammatical representation of certain
exemplary steps in identifying power line impedance values based
upon the circuitry of FIGS. 1 and 2;
[0016] FIG. 4 is a voltage waveform and switching waveform for a
solid state switch of the circuitry illustrated in FIG. 2 for
causing a voltage droop and a resonant ring used to identify
impedance parameters;
[0017] FIG. 5 is a detailed view of an exemplary resonant ring
caused in a voltage waveform and used for determine certain of the
impedance parameters in accordance with aspects of the present
technique;
[0018] FIG. 6 is graphical representation of a voltage waveform
similar to that of FIG. 4, before exemplary filtering of sampled
data;
[0019] FIG. 7 is a graphical representation of the waveform of FIG.
6 following high pass filtering of sampled data to flatten a
portion of the waveform around a resonant ring;
[0020] FIG. 8 is a more detailed illustration of the resonant ring
visible in FIG. 7 from which measurements can be made for computing
impedance parameters;
[0021] FIG. 9 is a graphical representation of an exemplary
frequency domain plot of the ring illustrated in FIG. 8;
[0022] FIG. 10 is a diagrammatical representation of an incident
energy measurement system in accordance with aspects of the present
technique;
[0023] FIG. 11 is a diagrammatical representation of an exemplary
system employing the incident energy measuring system;
[0024] FIG. 12 is a graphical representation of an exemplary
display of PPE levels based upon determinations made via the
systems of preceding figures;
[0025] FIG. 13 is a diagrammatical representation of an exemplary
MCC incorporating aspects of the present techniques;
[0026] FIG. 14 is a somewhat more detailed view of the exemplary
MCC of FIG. 13; and
[0027] FIG. 15 is a graphical representation of an exemplary
portable incident energy measurement device, again incorporating
aspects of the present techniques.
DETAILED DESCRIPTION
[0028] Turning now to the drawings, and referring first to FIG. 1,
an impedance monitoring system is illustrated and designated
generally by the reference numeral 10. The impedance monitoring
system is illustrated in a single-phase application. That is, the
system is illustrated for identifying the impedance of a
single-phase power source. As will be appreciated by those skilled
in the art, and as discussed in greater detail below, the system
may be easily adapted for identifying impedance parameters of
three-phase power lines and sources as well.
[0029] Impedance monitoring system 10 is illustrated as coupled to
a pair of power supply lines 12. Power supply lines 12, and any
upstream circuitry, such as transformers, connectors, and so forth
are considered to have a net impedance illustrated by equivalent
circuitry in box 14 of FIG. 1. The impedance 14 is, for the present
purposes, considered to be a collective or cumulative impedance of
the entire power supply network, represented generally by reference
numeral 16 to a point between a power supply grid and a load 18. As
discussed in greater detail below, the present system provides the
potential for determining impedance by measurement at or adjacent
to a load 18. In practical applications, the monitoring system 10
may be coupled at any point along the power supply lines.
[0030] Impedance 14 is generally considered to include inductive
components 20 and resistor components 22. The inductive and
resistive components will be present in both supply lines, although
for the present purposes these components may be grouped or
accumulated into a net inductive component and a net resistive
component as discussed in greater detail below.
[0031] System 10 includes line test circuitry 24 for perturbing the
voltage waveform transmitted through the power lines and for making
measurements of the voltage waveform. The line test circuitry 24 is
coupled to and works in conjunction with data processing circuitry
26. As discussed in greater detail below, the line test circuitry
24 and the data processing circuitry 26 may, in certain
applications, be analog circuitry, or at least partially comprise
analog circuitry. In a present embodiment, however, the line test
circuitry and the data processing circuitry digitally sample
voltage measurements and store the sampled data in a memory 28. The
stored sampled voltage measurements are then analyzed to determine
parameters of the voltage waveform that are used to compute the
values of inductive and resistive components of the line impedance.
As will be apparent to those skilled in the art, the data
processing circuitry 26 and memory 28 may be any suitable form. For
example, both of these components may be included in a general
purpose or application-specific computer. Moreover, the circuitry
may be local and permanently installed with an application, or may
be portable circuitry, such as in hand-held devices. Similarly, the
data processing circuitry and memory may be entirely remote from
the line test circuitry so as to provide the desired analysis
without necessarily displacing equipment or operators to the test
site.
[0032] The data processing circuitry 26 may be accessed and
interfaced with operator workstations by interface circuitry 30.
The interface circuitry 30 may include any suitable interfaces,
such as Ethernet cards and interfaces, Internet access hardware and
software, or other network interfaces. In appropriate situations,
the interface circuitry 30 may allow for interfacing with the data
processing circuitry by conventional serial port communication, and
so forth. As illustrated in FIG. 1, various operator interfaces may
be envisioned, including laptop computers, computer workstations,
and so forth, as represented generally by reference numeral 32 in
FIG. 1.
[0033] The line test circuitry 24 is illustrated in somewhat
greater detail in FIG. 2, along with the physical relationship
between the portions of the circuitry. As noted above, the
collective or cumulative impedance in the power lines may be
diagrammatically represented as a single inductive component 20 and
a resistive component 22. The line test circuitry 24 includes
voltage perturbation circuitry 33. The voltage perturbation
circuitry 33 includes a resistor 34 which is coupled in series with
a capacitor 36. A diode 35 and a solid state switch 38 are coupled
in parallel with the capacitor 36 so as to permit the capacitor 36
to be bypassed by creating a short circuit between the power lines
during a test sequence as summarized below. Those skilled in the
art will appreciate that the diode 35 may be omitted in direct
current embodiments. Where desired, an enable switch, represented
generally at reference numeral 40, may be provided in series with
these components. A switch 40 may permit an operator to enable a
test sequence, while removing the components from the power line
circuit during normal operation. Thus, switch 40 may permit any
leakage current between the power lines to be avoided.
[0034] Voltage measurement circuitry 42 is provided that spans the
power line conductors. The voltage measurement circuitry 42 may
include any suitable voltage measurement configurations, and is
particularly adapted to sample voltage across the power lines and
to provide values representative of the sampled voltage to data
processing circuitry 44. The data processing circuitry 44 includes
the data processing circuitry 26 and the memory 28 illustrated in
FIG. 1, along with any appropriate programming for carrying out the
functions, measurements, and analyses described below. To initiate
and advance the test sequences for measuring the parameters of the
power line impedance, the data processing circuitry 44 is coupled
to driver circuitry 46 which provides signals to solid switch state
38 to open and close the switch as described in greater detail
below.
[0035] Although the present invention is not intended to be limited
to any particular circuit configuration or component values, the
following component values have been found effective in identifying
impedance parameters in a 60 Hz power source. Resistor 34 was
implemented as a 1.OMEGA. resistor, while the value of capacitor 36
was 22 .mu.F. The switch 38 was selected as an insulated gate
bipolar transistor (IGBT) having a voltage rating of 1200V and
amperage rating of 400 A. It is advisable that the switch 38 be
overrated to some degree to permit peaks in the voltage waveform
that may result from opening and closing of the switch, and
particularly the affects of the resonant ring following
closure.
[0036] Exemplary logic 48 for a particular test sequence
implemented by the circuitry of FIG. 2 is illustrated
diagrammatically in FIG. 3. As noted above, voltage test circuitry
42 first begins to sample voltage across the power lines as
indicated at reference numeral 50. At a desired point in the
waveform, the switch 38 is closed, as indicated at step 52 in FIG.
3. Closure of switch 38 (with switch 40 closed to enable the
circuitry, where such a switch is provided) causes a short circuit
between the power lines, by routing current around capacitor 36.
The low value of the resistor 34 acts as a drain or burden, causing
a droop in the voltage waveform peak as described in greater detail
below. Subsequently, switch 38 is opened, as indicated at reference
numeral 54. Opening of the switch then causes a resonant ring
between the inductive component 20 of the line impedance and the
capacitor 36. This resonant ring is dampened by the resistive
component 22 of the power line impedance and by the resistor
34.
[0037] With the voltage continuously being measured (i.e., sampled)
by the voltage measurement circuitry 42, measurements are stored in
the memory circuitry for later analysis. In a present
implementation, with digital sampling of the voltage waveform, at
step 56 in FIG. 3, the voltage and ring parameters used to identify
the inductive and resistive components of the line impedance are
then determined. At step 58 the inductive and resistive components
of the line impedance are then computed based upon these identified
values.
[0038] Thus, with steps 50 through 58 being carried out, the system
response is measured continuously through the sampled data. These
measurements are summarized at step 60 in FIG. 3, where a value of
the voltage with the line test circuitry open is measured, and step
62 where a voltage across the power lines with the resistor 34 in
short circuit between the power lines is measured. Step 64
represents measurement of the ring parameters used in the
subsequent computations.
[0039] The calculations made of the inductive and resistive
components of the power line impedance in accordance with the
present techniques may be based upon the following computational
scheme. As will be appreciated by those skilled in the art, the
inductive-capacitive (LC) resonant frequency established upon
opening of switch 38, with little or no damping in the circuit may
be expressed by the relationship: 2 .times. .pi. .times. .times. f
= 1 LC .times. .times. load Equation .times. .times. 1 ##EQU1##
where f is the resonant frequency of the LC circuit, L is the value
of the inductive component of the line impedance, and the parameter
Cload is the value of the capacitor 36 discussed above.
[0040] It will be noted, however, the resistor 34, particularly
where a very low value of resistance is chosen, will provide
significant damping to the resonant ring. Indeed, as will be
appreciated by those skilled in the art, the value of the resistor
34 chosen strikes a balance between the desire to adequately sample
a voltage droop caused by the drain represented by the resistor,
while providing a significantly long (i.e., less damped) resonant
ring to permit measurement of the ring period or frequency.
Considering such damping, the relationship indicated in Equation 1
becomes described by the following relationship: 2 .times. .pi.
.times. .times. f = 1 LCload - ( R + Rload 2 .times. L ) 2 Equation
.times. .times. 2 ##EQU2## where the value R represents the value
of the resistive component of the line impedance, and the value
Rload represents the rating of the resistor 34 discussed above.
[0041] Based upon equation 2, and solving for the value of the line
inductance L, the following relationship may be expressed in terms
only of the values of Cload, Rload and the frequency f: L = 1 Cload
+ 1 Cload 2 - ( 2 .times. .pi. .times. .times. f ) 2 .times. Rload
2 2 .times. ( 2 .times. .pi. .times. .times. f ) 2 Equation .times.
.times. 3 ##EQU3##
[0042] To complete the system of equations desired for calculating
the inductive and resistive components of the line impedance, in
accordance with the present techniques, the voltage sag or droop
caused by closure of switch 38 and the presence of the drain or
burden resistor 34 may be expressed in terms of the voltage sampled
across the power lines with the line test circuitry open, indicated
by the quantity Vo, and the voltage across the power lines with the
circuitry closed, Vr, that is, with the resistor 34 in a series
across the power lines as follows: Vr = Vo .times. Rload j .times.
.times. 337 .times. L + R + Rload Equation .times. .times. 4
##EQU4## where Vo and Vr are either the peak or RMS ac voltage
values. It should be noted that the value 377 in Equation 4 (and in
the subsequent equations below) is derived from the product of
2.pi. and a line frequency of 60 Hz. As will be appreciated by
those skilled in the art, the equations will differ for other line
frequencies, although the principles for computation of the line
impedance parameters will remain unchanged. Equation 4 may be
rewritten as follows: Vr = Vo .times. Rload ( 377 .times. L ) 2 + (
R + Rload ) 2 Equation .times. .times. 5 ##EQU5##
[0043] It may be noted that Equation 5 may be solved in terms of
the value of the resistive component of the line impedance, R, as
follows: R = ( Vo .times. .times. Rload ) 2 - ( Vr .times. .times.
377 .times. L ) 2 Vr 2 - Rload Equation .times. .times. 6
##EQU6##
[0044] Thus, based upon three measured values alone, the values of
the inductive component of the line impedance, L, and the resistive
component of the power line impedance, R, may be computed by
Equations 3 and 6. The measured values, in accordance with the
present technique, are the values of Vo, Vr, and the frequency f,
or the period, which will be appreciated by those skilled in the
art, is the inverse of this frequency value.
[0045] FIG. 4 illustrates an exemplary ac voltage waveform and a
switching waveform for the switch 38 during an exemplary test
sequence in accordance with FIG. 3 to measure values for use in
calculating the impedance parameters in accordance with Equations 3
and 6 discussed above. FIG. 4 illustrates the waveforms graphically
as indicated generally by reference numeral 56. The voltage
waveform is illustrated graphically with respect to voltage, as
indicated axis 68 over time, as indicated by axis 70. The voltage
trace 72 of the waveform takes the form of a sine wave. Trace 74
represents the state of switch 38 or, more particularly, the signal
applied to drive the gate of the switch to change its conductive
state during the testing procedure.
[0046] As can be seen from FIG. 4, once sampling of the waveform
has begun, samples will be taken continuously at a desired
frequency, above the Nyquist rate, to provide reliable data for
analysis. In a first cycle 76 of the waveform, with the test
circuit open, a peak voltage 78, corresponding to Vo will be
detected, among the other values detected and stored. At some point
during or preceding a second cycle 82, switch 38 is placed in a
conductive state to complete the current carrying path between the
line conductors. The change in state of the switch is indicated at
the rising edge 74 of the waveform, and occurs at time 80. As a
result of the significant voltage drain caused by resistor 34, a
sag or droop is detected in the peak 84 of the voltage waveform,
with the peak generally corresponding to the value Vr.
Subsequently, the switch 38 is opened, as indicated by the drop in
waveform 74 that occurs at time 86 indicated in FIG. 4. The opening
of switch 38 causes a resonant ring as indicated generally at
reference numeral 88. As noted above, the resonant ring may have a
peak voltage above the peak voltage of the sinusoidal waveform, and
the switch 38 may be selected to accommodate such peaks.
[0047] FIG. 5 illustrates a more detailed view of the resonant ring
occurring from opening of the switch of the line test circuitry.
Again graphed with respect to a voltage axis 68 and a time axis 70,
the ring occurs as the voltage across the lines is decreasing, as
indicated by the initial slope of trace 72. The falling edge of
waveform 74 represents the removal of the drive signal to the
switch causing opening of the circuit. The resulting resonant ring
88 will have a period, or consequently a frequency, that is a
function of the circuit component parameters and of the inductive
component of the line impedance. Because the voltage waveform is
continuously sampled, the frequency or period may be measured, with
a full period being indicated by reference numeral 90 in FIG. 5. As
will be apparent to those skilled in the art, the period may be
measured in a number of ways, as may the frequency. For example, a
half cycle alone may be used to determine the frequency or period,
or a full or even more than one cycle may be used. Similarly, in a
present embodiment, the values of the ring as sampled by the
circuitry may be compared or processed with the naturally declining
value of the sinusoidal waveform to provide an accurate indication
of the period of the resonant ring. Based upon the measured
voltages, Vo, Vr and either the period of the resonant ring or its
frequency, then, Equations 3 and 6 may be employed or determining
the values of L and R.
[0048] An alternative approach to identifying the parameters
discussed above is illustrated in FIGS. 6-9. As illustrated in FIG.
6, the voltage waveform that is sampled by the voltage measurement
circuitry may be illustrated as having successive cycles 76 and 82,
with a voltage droop or sag occurring in cycle 82 due to the
resistor 34 discussed above. The subsequent ring upon a removal of
the short circuit across the power lines is again indicated at
reference numeral 88. The data may be high-pass filtered to
generally flatten the waveform as indicated at reference numeral 92
in FIG. 7. The high-pass filtered waveform will then exhibit the
ring which may be timed to occur during a generally linear portion
of the sine wave, as indicated at reference numeral 94 in FIG. 7.
From the data, the ring 94 may be extracted as indicated generally
in FIG. 8. The period, or half period, or frequency of the ring may
then be determined, as indicated at reference numeral 90 in FIG. 8.
Finally, where desired, the waveform may be converted by a
one-dimensional fast Fourier transform to a frequency response
relationship as indicated in FIG. 9. This frequency response may be
represented graphically along an amplitude axis 98 and a frequency
axis 100. The frequency trace 102 in FIG. 9 indicates a resonant
frequency at peak 104 which generally corresponds to the wavelength
measured for the resonant ring as discussed above. As noted, either
the frequency or the period of the waveform may be used for the
calculation of the impedance parameters.
[0049] FIG. 10 is a diagrammatical representation of an incident
energy measurement system 106 in accordance with aspects of the
present technique. The incident energy measurement system 106
comprises modules represented by blocks 10, 108, 110, 112, 114,
116, and 118. The modules (blocks 10 and 108-118) may be hardware,
software, firmware, or some combination of hardware, software, and
firmware. Additionally, an individual module does not necessarily
solely comprise each illustrated module function. The modules shown
in the blocks 10 and 108-118 are merely one example and other
examples can be envisaged wherein the functions are distributed
differently or where some modules are included and other modules
are not included. Further, FIG. 10 also illustrates inputs 120 and
outputs 122 from each of the modules 10 and 108-118. Those of
ordinary skill in the art will appreciate that the inputs 120 and
the outputs 122 are exemplary. In alternate embodiments, the inputs
120 and the outputs 122 to each of the modules 10 and 108-118 may
differ. Similarly, while FIG. 10 call for certain "inputs," in many
installations, the values used in the various determinations will
be known in advance, may be pre-programmed in the modules, or may
be provided in a menu for user selection. Lastly, while the
incident energy measurement system 106 is described herein in
regard to a three phase system, those of ordinary skill in the art
will appreciate that the techniques herein may be applied to single
phase or dual phase systems.
[0050] As described above, the impedance monitoring module 10
(previously referred to as the impedance monitoring system 10) may
receive or calculate a Cload value 124 (e.g., the value of the
capacitor 36, described above), and an Rload value 126 (e.g., the
rating of the resistor 34, described above), a resonant ring
frequency f 128, a voltage Vo 129 (the voltage sampled across the
power line with the line test circuitry opened), and a voltage Vr
132 (a voltage across the power line with the circuitry closed) for
each phase of a three phase power transmission system. From these
inputs, as described in relation to Equations 1-6 above, the
impedance monitoring module 10 may compute an inductive component
of the line impedance (L) 134 and a resistive component of the line
impedance (R) 136. As illustrated in FIG. 10, the inductive
component 134 and the resistive component 136 may be either
communicated by the system 106 as outputs, transmitted to the
bolted fault current calculation module 108, or both
[0051] The bolted fault current calculation module 108 may
calculate a bolted fault current (Ibf) 138 using Ohm's law for AC
circuits, which is: Ibf = V Z Equation .times. .times. 7 ##EQU7##
where Ibf is the bolted fault current 138, V is the three phase
system voltage Vs 130 calculated based on the voltage Vo 129 for
each of the three phases, and Z is a line impedance calculated from
the inductive component 134 and the resistive component 136 for
each of the three phases. As illustrated in FIG. 10, the bolted
fault current 138 may either be communicated by the system 106,
transmitted to the arc current calculation module 110, or both
[0052] The arc current calculation module 110 calculates an arc
current Ia 144. In one embodiment, the arc current calculation
module 110 calculates the arc current Ia 144 based on the equations
set forth in the Institute for Electrical and Electronics Engineers
("IEEE") Guide for Performing Arc-Flash Hazard Calculations, IEEE
Std. 1584 (2002), which is hereby incorporated by reference.
Specifically, if the voltage Vo is less than 1000 volts, the arc
current 144 may be calculated as follows: log(Ia)=K1+0.662
log(Ibf)+0.966V+0.000526G+0.5588V(log(Ibf)-0.00304G(log(Ibf))
Equation 8 where the arc current 144 equals 10.sup.log(Ia); K1
equals -0.153 for open configurations (i.e., configurations where
the conductors that may arc are not contained within a chassis or
enclosure) and -0.097 for enclosed configurations; V is the voltage
Vs 130 in kilovolts (KV), G is a gap 142 between the conductors
that could potential arc in millimeters (mm); and Ibf is the bolted
fault current 138 in kiloamps (KA). If the voltage V is greater
than 1000 volts, the following equation is used instead of Equation
8: log(Ia)=0.00402+0.983 log(Ibf). Equation 9
[0053] Those of ordinary skill in the art will appreciate that
Equations 8 and 9 are derived empirically using statistical
analysis and curve fitting programs. As such, they are only
applicable for voltages Vs 130 in the range of 208V-15,000V, power
line frequencies of either 50 Hz or 60 Hz, bolted fault currents
138 in the range of 700 A-106,000 A, and for gaps between
conductors 142 of 13 mm to 152 mm. In embodiments incorporating
parameters outside these ranges, the theoretically-derived Lee's
method can be used in place of the modules 110 and 112, as will
described further below.
[0054] Once calculated, the arc current 144 may either be
communicated by the system 106, transmitted to the normalized
incident energy calculation module 112, or both. In one embodiment,
the normalized incident energy calculation module 112 calculates a
normalized incident energy 148 using the following equation:
log(En)=K2+K3+1.081 log(Ia)+0.0011G Equation 10 where
10.sup.log(En) is the normalized incident energy 148 in joules per
square centimeter (J/cm.sup.2); K2 is -0.792 for open
configurations and -0.555 for enclosed configurations; K3 is zero
for ungrounded or high-resistance grounded systems and -0.113 for
grounded systems; Ia is the arc current 144; and G is the gap 142.
Equation 10 calculates the incident energy normalized for an arc
time t of 0.2 seconds and a distance from the possible arc point
(e.g., the MCC) to a measurement point of 610 mm. One of ordinary
skill in the art will appreciate that in alternate embodiments, the
normalized incident energy calculation module 112 may employ an
alternate version of Equation 10 that has been normalized for a
different arc time t or a different distance D. Further, as
described above, for embodiments where one of the parameters Vo, f,
Ibf, or G falls outside of the empirically tested range (see
above), the normalized incident energy calculation module 112 may
be absent from incident energy measurement system 106.
[0055] Once calculated, the normalized incident energy 148 may
either be communicated by the system 106, transmitted to the
incident energy calculation module 114, or both. Unlike the
normalized incident energy calculation module 112 which is
normalized for a distance of 610 mm, the incident energy
calculation module 114 is configured to calculate an incident
energy 156 in J/cm.sup.2 at an arbitrary distance D 152 from the
point of the potential arc. For example, the incident energy
calculation module 114 can calculate the incident energy 156 at one
meter from an MCC or three meters, and so forth. In one embodiment,
the incident energy calculation module 114 calculates the incident
energy 156 with the following equation: E = 4.184 .times. ( Cf )
.times. ( En ) .times. ( t 0.2 ) .times. ( 610 x D x ) Equation
.times. .times. 11 ##EQU8## where E is the incident energy 148; Cf
is 1.0 if the voltage Vs 130 is greater than 1000 volts and 1.5 if
the voltage Vs 130 is equal to or less than 1000 volts; En is the
normalized incident energy 148; t is an arc time 150, which is
described in greater detail below; D is the distance 152; and x is
a distance exponent, which is also described in greater detail
below.
[0056] As noted above, one of the elements in Equation 11 is the
arc time 150. The arc time 150 is a factor in calculating the
incident energy 156, because the amount of energy generated by the
arc flash is proportional to the length of time that the arc
current 144 is actually flowing (i.e., the device is arcing). Those
of ordinary skill in the art will appreciate that in most
situations devices such as circuit breakers or fuses will detect
the sudden increase in current that accompanies an arc flash and
interrupt power to the system. Unfortunately, these devices are not
instantaneous, and the arc current 144 will flow for approximately
as long as it takes the devices to activate and interrupt the
power. Equation 10 above (the normalized incident energy
calculation) is normalized for an arc time of 0.2 seconds (i.e.,
the arc current 144 flows for 0.2 seconds). However, the arc time
150 for a particular system may not be 0.2 seconds. As such, one of
the inputs to the incident energy calculation module 114 may be the
arc time 150 for the system of interest.
[0057] Those of ordinary skill in the art will appreciate that
there are several techniques for determining the arc time 150. For
example, the manufacturer of an electrical system, such as a
circuit breaker, a fuse, or a circuit interrupter, may provide the
arc time 150 for the electrical system. Also, IEEE Std. 1584
provides versions of Equation 11 pre-calculated with the arc times
150 for a variety of standard types of fuses or circuit breakers.
Further, the arc time 150 may also be determined by charting the
time/current characteristics for the circuit breaker or fuse that
will sever the electrical connection and stop the flow of the arc
current 144.
[0058] As described above, Equation 11 also employs the distance
exponent x. The distance exponent x may be determined using a
look-up table, such as Table 1 (below) from IEEE Std. 1584 where an
equipment type 154 (open air, switchgear, MCC and panels, or cable)
is entered in the incident energy calculation module 114 as an
input. TABLE-US-00001 TABLE 1 Vs Equipment Type Distance Exponent x
208 V-1000 V Open Air 2.0 Switchgear 1.473 MCC and panels 1.641
Cable 2.0 1000 V-5000 V Open Air 2.0 Switchgear 0.973 Cable 2.0
5000 V-15,000 V Open Air 2.0 Switchgear 0.973 Cable 2.0
In one embodiment, the incident energy calculation module 114 is
programmed with a look-up table (LUT) comprising Table 1.
[0059] In an alternate embodiment of the incident energy
measurement system 106, the incident energy calculation module 114
calculates the incident energy using Lee's method. As stated
earlier, Lee's method is theoretical and, thus, can be used to
calculate the incident energy 156 outside the empirical range of
Equations 8-11; (i.e., Vs greater than 15,000V, arc fault currents
greater than 106,000 A, gaps between conductors larger than 152 mm,
and so forth). As Lee's method is based on the arc fault current
138, the modules 112 and 114 may be omitted from an embodiment of
the incident energy measurement system 106 that employs Lee's
method. The incident energy calculation module 114 can use the
following equation to calculate the incident energy 156 using Lee's
method: E = 2.142 * 10 6 .times. ( V ) .times. ( Ibf ) .times. ( t
D 2 ) Equation .times. .times. 12 ##EQU9## where E is the incident
energy 156 measured in J/cm.sup.2; V is the voltage Vs 130; t is
the arc time 150 in seconds; D is the distance 152; and Ibf is the
arc fault current 138. Once the incident energy calculation module
114 has calculated the incident energy 156 at the distance 152 this
value may be reported by the incident energy measurement system
106.
[0060] As illustrated in FIG. 10, the incident energy measurement
system 106 may also comprise the flash protection boundary
calculation module 116. The flash boundary calculation module 116
may operate in conjunction with or in alternative to the incident
energy calculation module 114. As its title suggests, the flash
protection boundary calculation module 116 calculates a flash
protection boundary 160 for the system being measured (e.g., for an
MCC). Those of ordinary skill in the art will appreciate that the
incident energy decreases proportionally as the distance from the
arcing point increases. At some distance from the origination point
of the arc, the incident energy is low enough to be considered
acceptable. This distance is known as the flash protection boundary
160. In one embodiment, the flash protection boundary 160 is deemed
to exist at a distance from the arcing point where an incident
energy Eb 158 is equal to 5.0 J/cm.sup.2.
[0061] The flash protection boundary calculation module 116 is
similar to the incident energy calculation module 114 except that
rather than determining the incident energy 156 at the distance
152, the flash protection boundary calculation module 116
determines the flash protection boundary (i.e., a distance) where
the incident energy 156 will be at the incident energy level Eb
158. In one embodiment, the flash protection boundary calculation
module 116 employs the following equation: Db = 4.184 .times. ( Cf
) .times. ( En ) .times. ( t 0.2 ) .times. ( 610 x Eb x ) 1 x
Equation .times. .times. 13 ##EQU10## where Db is the flash
protection boundary 160 in millimeters; Cf is 1.0 if the voltage Vs
130 is above 1000 volts and 1.5 if the voltage Vs 130 is equal to
or less than 1000 volts; t is the arc time 150; Eb is the incident
energy 158 at the flash protection boundary 160 (e.g., 5.0
J/cm.sup.2); and x is the distance exponent, as described above.
Once calculated, the flash protection boundary 160 can be reported
by the incident energy measurement system 106.
[0062] The flash protection boundary 160 can also be determined
using Lee's method. Specifically, the flash protection boundary
calculation module 116 may employ the following equation: Db =
2.142 * 10 6 .times. ( V ) .times. ( Ibf ) .times. ( t Eb 2 )
Equation .times. .times. 14 ##EQU11## where Db is the flash
protection boundary 160; V is the voltage Vs 130; Ibf is the arc
fault current 138; t is the arc time 150; and Eb is the incident
energy 158.
[0063] Alternatively or in conjunction with the incident energy
calculation module 114 and the flash protection boundary
calculation module 116, the incident energy measurement system 106
may also include a PPE level calculation module 118. In one
embodiment, the PPE level calculation module 118 calculates the PPE
level 161 at one or more distances 152 from the potential arcing
point. For example, the PPE level calculation module 114 may
calculate that level 1 PPE is appropriate at six meters from the
potential arc point or that level 3 PPE is appropriate for work
being performed on equipment in the same MMC as the potential
arcing point. To determine the PPE level 161, the PPE level
calculation module 118 may employ either Equation 11 or 12, as
outlined above, in conjunction with the following table from NFPA
70E, which is hereby incorporated by reference: TABLE-US-00002
TABLE 2 PPE Category Eb (in J/cm.sup.2) 0 <5.0 1 5.0-16.74 2
16.74-33.47 3 33.47-104.6 4 104.6-167.36
[0064] For example, the PPE level calculation module 118 may
compute an incident energy of 18.24 J/cm.sup.2 at 0.1 meters using
Equation 11, as described above. Because 18.24 J/cm.sup.2 falls
between 16.74 and 33.47, the PPE level calculation module 118 would
determine that level 2 PPE is appropriate at 0.1 meters from the
arc point. Once the PPE level calculation module 118 determines the
PPE level 160, it may communicate this determination out of the
incident energy measurement system 106 for display, as further
described below. In one embodiment, Table 2 is stored in the PPE
level calculation module 118 as a look-up table.
[0065] FIG. 11 is a diagrammatical representation of an exemplary
system 162 employing the incident energy measuring system 106. The
system 162 comprises a three phase power bus 164 and a data bus
166. The three phase power bus may be coupled to the power supply
grid 16. As illustrated, the three phase power bus 164 provides
three phase power to bus bars 165 within MCCs 170 and 172. This
form of power distribution is well known to those of ordinary skill
in the art and need not be described in greater detail. The data
bus 166 provides a communication pathway between a remote command
and control unit 168 and the MCCs 170 and 172. As will be described
in greater detail below, the illustrated system 162 includes two
types of exemplary MCCs 170 and 172. The MCC 170 comprises a
distributed incident energy monitoring system 106, while the MCC
172 comprises a stand-alone incident energy monitoring system 106.
Those of ordinary skill in the art will appreciate that the MCCs
170 and 172 are exemplary. In alternate embodiments, virtually any
type of electrical device or apparatus suitable for use with the
incident energy monitoring system 106 may employ the incident
energy monitoring system 106 in the manner described below.
[0066] The control unit 168 may comprise a network interface 174,
incident energy calculation circuitry 176, memory 178, and a
computer 180. The network interface 174 facilitates communication
between a control center or remote monitoring station and the MCCs
170 and 172. In one embodiment, the control unit 168 communicates
with the voltage measurement circuitry 42 disposed on the MCC 170
via the network interface 174. The incident energy calculation
circuitry 176 comprises the data processing circuitry 44 and some
or all of the modules 108-118. The incident energy calculation
circuitry 176 receives the inputs 120 (see FIG. 10) from the
voltage measurement circuit 42 disposed in the MCC 170 or from a
memory 178, which is programmed with some or all of the inputs 120.
The incident energy calculation circuitry 176 employs the inputs
120 to produce the outputs 122, as described above with regard to
FIGS. 2 and 10. In one embodiment, the control unit 168 is
configured to trigger the measurement of the incident energy 156 or
any of the other outputs 122. In another embodiment, the outputs
122, once calculated, are transmitted to the computer 180 within
the control unit 168. In still another embodiment, the computer 180
may be employed to program the memory 178 with the inputs 120.
[0067] As stated above, the MCC 170 includes a distributed version
of the incident energy monitoring system 106. In this embodiment,
the MCC 170 works in conjunction with the control unit 168 to
determine the outputs 122, as described above. As such, the MCC 170
comprises only a portion of the incident energy monitoring system
106--namely, the voltage measurement circuitry 42 and the voltage
perturbation circuitry 33. The voltage perturbation circuitry 33
and the voltage measurement circuitry function as described above
in regard to FIG. 2 except that the voltage perturbation circuitry
33 and the voltage measurement circuitry communicate with the data
processing circuitry 44 within the incident energy calculation
circuitry 176 via a network interface 182 and the data bus 166. In
other words, circuitry within the MCC 170 is configured generate
the resonant ring 88 (see FIG. 4), to measure the resonant ring 88,
and to communicate the measurements to the incident energy
calculation circuitry 176, which determines the outputs 122. In one
embodiment, the control unit 168 may transmit a portion of the
outputs 122 back to the MCC 170 for display on a display 184, as
will be described further below in regard to FIG. 12. Those skilled
in the art will appreciate that in alternate embodiments, the
system 162 may comprise multiple MCCs 170 each of which is
supported by a single control unit 168. Further, in still other
embodiments, some of the modules 108-118 may be located in the MCC
170 instead of the control unit 168. Moreover, in yet other
embodiments, the MCC 170 may be configured to process the voltage
measurements from the voltage measurement circuitry prior to
communicating with the data processing circuitry 44.
[0068] Turning next to the stand-alone MCC 172, the MCC 172
comprises the incident energy monitoring system 106. As
illustrated, the incident energy measurement system 106 is coupled
to the three phase power bus 166 via the bus bars 165. As such, the
impedance monitoring module 10 within the incident energy
monitoring system 106 can function as described above in regard to
FIGS. 1-10. The MCC 172 also comprises the memory 178, which can
provide the inputs 120 to the incident energy measurement system
106. The memory 178 may either be programmed by a computer coupled
directly to the MCC 172 (not shown) or by the computer 180 in the
control unit 168 via the data bus 166 and a network interface 186.
The incident energy monitoring system 106 within the MCC 172 may be
configured to display one or more of the outputs 122 on the display
184, as described further below. In addition, in one embodiment,
the MCC 172 is configured to transmit one of more of the outputs
122 to the control unit 168 via the data bus 166.
[0069] Those skilled in the art will appreciate that the data bus
166 and the network interfaces 174, 182, and 186 may employ a wide
variety of suitable communication technologies or protocols. For
example, in one embodiment, the data bus 166 may comprise a local
area network, and the network interfaces 174, 182, and 186 may
comprise network interface cards. In yet another example, the data
bus 166 may comprise a wireless network based on the IEEE 802.11
standard or another suitable wireless communication protocol, and
the network interfaces 174, 182, and 186 may comprise wireless
transmitters and receivers.
[0070] FIG. 12 is a graphical representation of an exemplary
display 184 of PPE levels based upon determinations made via the
systems of preceding figures. Both the MCC 170 and the MCC 172 may
include the display 184 to display one or more of the outputs 122
to a user or operator. The illustrated display 184 comprises a set
of PPE level indicator lights 188. In one embodiment, the display
184 may be mounted in a front panel of the MMC 170 or 172. The PPE
level indicator lights 188 may be configured to display the level
of PPE appropriate for performing maintenance on the MMC 170 or
172. Specifically, a particular light corresponding to the PPE
level may be illuminated. In an alternate embodiment, a light tower
or siren-style light mounted to the MCC 170 or 172 may produce
colored light indicative of the PPE level. The display 184 may also
comprise a screen 190. In one embodiment, the screen 190 comprises
a liquid crystal diode (LCD) display or other form of textual or
graphical display. The screen 190 is configurable to display any of
the outputs 122. For example, the screen 190 may be configured to
display the flash protection boundary 160 or the normalized
incident energy 148. As will be appreciated by those skilled in the
art, the determination of the PPE level, and its display at or near
the point of entry for maintenance can greatly facilitate the task
of donning the correct PPE prior to servicing of the equipment.
[0071] FIG. 13 is a diagrammatical representation of an exemplary
MCC 172 incorporating aspects of the present techniques. The MCC
172 comprises a chassis 192, an incident energy measurement bucket
194, and a plurality of motor control buckets 196. As described
above, the MCC 172 comprises the bus bars 165, which are coupled to
the three phase power bus 164. While not illustrated in FIG. 13,
the data bus 166 may also be connected to the MCC 172. Similarly,
as will be appreciated by those skilled in the art, in practice,
the bus bars 165 may be disposed behind one or more plates or
barrier
[0072] The incident energy measurement bucket 194 comprises the
incident energy measurement system 106. Those of ordinary skill in
the art will appreciate that there is an intrinsic resistance in
the connection between the voltage perturbation circuit 33 and the
bus bars 165. This intrinsic resistance can affect the accuracy of
the measurements of the voltage measurement circuitry 42. To reduce
the effects of this intrinsic resistance, the voltage measurement
circuitry 42 may be coupled to a first set of stabs 198a and the
voltage perturbation circuitry 33 may be coupled to a second,
separate set of stabs 198b. Those of ordinary skill in the art will
appreciate that coupling the voltage measurement circuitry 42 to a
different set of stabs than the voltage perturbation circuitry 33
increases the accuracy of the measurement of the voltage
measurement circuit 42. That is, due to the significant current
draw of the voltage perturbation circuitry 33, voltages that would
be measured at that circuit could be significantly affected by the
resistance of that circuit's stabs, fuses, and so forth. On the
other hand, the current draw of the voltage measurement circuitry
42 is negligible. The parallel connection of the two circuits,
then, allows for more accurate measurements of the voltages during
tests.
[0073] FIG. 14 is a somewhat more detailed view of the exemplary
MCC 172. For simplicity, like reference numerals have been used to
designate features previously described in reference to FIG. 13. In
the embodiment illustrated in FIG. 14, the first set of stabs 198a
are configured to be coupled to the bus bars 165 at a location
above (i.e., electrically closer to the three phase power bus 164).
While not illustrated in FIG. 13 or 14, the incident energy
measurement bucket may also comprise the display 184, the memory
178, and the network interface 186, as illustrated in FIG. 11.
[0074] FIG. 15 is a graphical representation of an exemplary
portable incident energy measurement device 200, again
incorporating aspects of the present techniques. In one embodiment,
the portable device 200 comprises a laptop, tablet, or a portable
computer system. In another embodiment, the portable device 200
comprises a personal digital assistant or palm-top computer system.
The portable device 200 comprises the incident energy measurement
system 106 (not shown) and a plurality of test leads 202a, 202b,
and 202c. The test leads 202a-c are configured to be connected to a
power source, such as the three phase power bus 164. In the
illustrated embodiment, the test leads 202a-c comprise clips to
connect the test leads to the three phase power bus 164. In
alternate embodiments, the test leads 202a-c may comprise any form
of connector suitable for connecting the test leads 202a-c to a
power source.
[0075] In the embodiment illustrated in FIG. 15, each of the test
leads 202a-c are coupled to both the voltage perturbation circuitry
33 and the voltage measurement circuitry 42, described above. In
alternate embodiments, the portable device may comprise an
additional set of test leads 202a-c that are connected to the
voltage measurement circuitry 42 to increase the accuracy of the
portable device 200, as described above in regard to FIGS. 13 and
14. The portable device 200 may also contain a power source, such
as a battery or an ac plug (not shown).
[0076] The portable device may also comprise a display 204 and an
input device 206. In one embodiment, the display 204 is a liquid
crystal diode (LCD) display. The input device 206 may be an
internal keyboard, an external keyboard, or a touch screen. In one
embodiment, the display 204 and the input device 206 comprise a
single touch screen. The portable device 200 may also comprise a
communication interface 208 for connecting the portable device 200
to a computer system. The communication interface 208 may employ
any communication protocol suitable for communication between the
portable device 200 and the computer. For example, the
communication interface 208 may comprise a USB port, a Firewire
port, an Ethernet port, a Bluetooth transmitter and receiver, an
802.11 transmitter and receiver, and so forth.
[0077] In operation, the operator of the portable device 200
connects the test leads 202a-c to each of the phases of the three
phase power bus 164. Because the portable device 200 is
preprogrammed with the Cload value 124 and the Rload value 126, the
impedance monitor module 10 (not shown) within the portable device
200 can compute the inductive component of the line impedance 134
and the resistive component of the line impedance 136 for each of
the phases of the three phase power bus 164, as described above in
regard to FIG. 1-9. These values (134 and 136) can then be
displayed on the display 204.
[0078] In addition, the incident energy measurement system 106
within the portable device 200 can also determine the bolted fault
current 138, the arc current 144, the normalized incident energy
148, the incident energy 156, the flash protection boundary 160,
and/or the PPE level 161, as described above in reference to FIG.
10. In one embodiment, the inputs 120 (FIG. 10) may be entered into
the device 200 via the input device 206. In another embodiment, the
inputs 120 may be stored on a memory (not shown) within the
portable device 200. In either case, the incident energy
measurement system 106 within the portable device 200 determines
one or more of the outputs 122 and then transmits the outputs 122
to the display 204.
[0079] Many of the modules described above with reference to FIG.
10 may comprise an ordered listing of executable instructions for
implementing logical functions. These ordered listing can be
embodied in a computer-readable medium for use by or in connection
with a computer-based system that can retrieve the instructions and
execute them to carry out the previously described processes. In
the context of this application, the computer-readable medium can
be a means that can contain, store, communicate, propagate,
transmit or transport the instructions. By way of example, the
computer readable medium can be an electronic, a magnetic, an
optical, an electromagnetic, or an infrared system, apparatus, or
device. An illustrative, but non-exhaustive list of
computer-readable mediums can include an electrical connection
(electronic) having one or more wires, a portable computer
diskette, a random access memory (RAM) a read-only memory (ROM), an
erasable programmable read-only memory (EPROM or Flash memory), an
optical fiber, and a portable compact disk read-only memory
(CDROM). It is even possible to use paper or another suitable
medium upon which the instructions are printed. For instance, the
instructions can be electronically captured via optical scanning of
the paper or other medium, then compiled, interpreted or otherwise
processed in a suitable manner if necessary, and then stored in a
computer memory.
[0080] While only certain features of the invention have been
illustrated and described herein, many modifications and changes
will occur to those skilled in the art. It is, therefore, to be
understood that the appended claims are intended to cover all such
modifications and changes as fall within the true spirit of the
invention.
* * * * *